Ed Miliband is facing the possibility of losing Diane Abbott, one of his most prominent frontbenchers, if he supports military intervention in Syria.

Abbott, the shadow public health minister, said she would be "in a very difficult position" if Labour backed missile strikes on the wartorn country, as she had not yet seen any evidence that would justify getting involved in the conflict.

Parliament will reconvene on Thursday to debate the Assad regime's alleged use of chemical weapons in Ghouta, east of Damascus, and the coalition is likely to recommend that Britain supports "surgical" military strikes led by the US.

Labour has not decided whether to support any such move, but Miliband met Cameron on Tuesday afternoon to consider the options.

Abbott told the Guardian she had not made up her mind before the parliamentary debate but she was currently opposed to intervention based on the available evidence. She is the first Labour frontbencher to say she is not in favour of Britain getting involved in the Syrian conflict without UN backing.

"I voted against the Iraq war. At the moment I can't see anything that would make me vote for intervention in Syria," she said. "On the basis of what I've heard so far, I would be opposed to a military intervention. Essentially it's a civil war. What Libya and Egypt have taught us is that these situations in the Middle East are complex. It's not good guys in white hats and bad guys in black hats."

Asked whether she would resign from the frontbench if Labour supported military action, Abbott said: "It would put me in a difficult position."

She quoted Colin Powell, the former US secretary of state, whose "Pottery Barn rule" warns that "you broke it, you own it". She said: "If you make a military intervention in Syria, you have to take ownership of the situation. I think it would be wrong … I wouldn't support anything that wasn't legal, and the only legal way would be UN backing."

She said Tony Blair's decision to join the "clamour" for an attack on Syria was "another reason why it's probably a bad idea".

Abbott is an influential figure within her party and could sway others to oppose military intervention. If Labour were to support the coalition in Syria, her comments could have echoes of Clare Short's opposition to the first Gulf war. Short resigned as a shadow Labour minister in 1991 after she criticised the party's support for the allied bombing campaign.

Labour sources said the party still had many questions about whether Britain should join in with strikes or other action against Syria, particularly over its legality and the overall strategy.

On Tuesday morning Douglas Alexander, the shadow foreign secretary, said Labour would not write a "blank cheque" of support and urged the government to make a proper legal case for intervention.

"There are very legitimate issues of real concern which I think we've got an obligation to be asking the government in these critical days," Alexander told BBC News. "I'm not ruling out the possibility that Labour could support the government, but I'm certainly not prepared to write the government a blank cheque."

It is understood Labour will make a final decision on Thursday, shortly before the parliamentary debate. MPs will be whipped, meaning they have to vote in line with the position of their leaders. Frontbenchers including Abbott will come under pressure to vote with their leader or resign.